The 2.5mm eyepieces provide a natural 1.4x progression in power from the 3.5mm.
You don't have to sacrifice field in order to gain sharpness and power.
And, you're not limited to slow scopes, or have to squint through tiny eye lenses.
These are the lunar and "planetary" eyepieces with field to spare.

The 7-element, Nagler Type
6 design is based on the original patented Nagler design. This new 7-element
design update for the short focal length Nagler deserves its own designation.
Using different exotic materials, coating processes and design ingenuity, Al
has massaged more eye-relief, contrast, and true field of view into the shorter
focal length Naglers, while maintaining the absolute sharpness of the originals.
Eye-relief is 12mm on each model, and all are sized about the same as the original
7mm! Weight about 0.4 to 0.5 lbs.

Concept:
The goal we set designing
these new eyepieces was to increase contrast, maintain or increase eye-relief,
reduce size and weight, and maintain the sharpness of the originals. They are
all parfocal and great with the Bino Vue.

Why Are There 3 Types of Naglers?

To achieve the full magnification range over 82° apparent field of view requires two types: Nagler Type-5 with focal lengths of 31, 26, 20, 16mm and Nagler Type-6 with focal lengths of 13, 11, 9, 7, 5, 3.5, 2.5mm.

The Type-5 permits the largest true field possible (31mm has 42mm field stop), while the shorter focal length Type-6 models are parfocal, with similiar size and weight and all with a comfortable 12mm eye relief.

The Type-4 models feature about 18mm of eye relief and with the "instajust" eyeguard are best for eyeglass wearers or Dioptrx users in their 22, 17, 12mm focal length range.